A 6 year old girl from Medellin, Colombia, South America, presented with twenty-five days of left-sided headache, vomiting, and episodes of syncope lasting roughly five seconds. The patient had no other medical history of significance. She had no café au lait spots, and no other tumors or masses in other body sites. Neurological examination revealed no focal deficits. An MRI scan showed a large intra-axial left parietal lobe mass with significant surrounding edema. The mass did not appear to originate from the dura; it did reach or breach the pia on the parietal convexity, and medially close to the falx. The falx was pushed by the mass effect of the tumor and was clearly separate from it. (Fig 1). A surgical excision was done in Colombia, and a tentative diagnosis made. The slides and selected blocks were sent to the University of Missouri for consultation. Since the time of surgery, the patient has been treated with radiation (thirty treatments) and has done extremely well. Neurologically she is almost completely normal, and her mother reports that the patient is asymptomatic and leading a normal life attending school.

PATHOLOGY

The resected soft tissue fragments were grossly hemorrhagic, pink and yellow. Histologically, the tumor was composed of sheets of spindle cells, with very little in the way of distinctive patterns. Some of the larger pieces had alternating dense and hypodense fascicles arranged in vague marbled or herringbone patterns. (Figs 2 and 3) The tumor was highly cellular with small foci of necrosis. (Fig 4) Adjacent gliotic brain was sharply-demarcated from the tumor, without any single-cell (glioma-like) invasion. The tumor was well vascularized but had no vascular hyperplasia and did not have a hemangiopericytoma-like staghorn vascular pattern. The neoplastic cells had large fusiform nuclei with areas of marked pleomorphism; most nuclei showed clumped chromatin and prominent nucleoli. (Fig 5) Mitotic activity was brisk, with a Ki-67 proliferation index of at least 75%, approaching 100% in certain areas. (Fig 6) The tumor cells were strongly immunopositive for Vimentin and p53 (Figs 7 and 8); a CD56 (Neural Cell Adhesion Molecule, NCAM) immunostain was diffusely moderately positive (Fig 9). The tumor was negative for Smooth Muscle Actin, Skeletal Muscle Actin, S100 protein, Desmin, Glial Fibrillary Acidic Protein (GFAP), Synaptophysin, Chromogranin, Neuron-Specific Enolase (NSE), Epithelial Membrane Antigen (EMA) and Cytokeratin (CK). A CD34 immunostain highlighted endothelial cells in tumor blood vessels, but not the tumor cells themselves (Fig 10). Additional immunostains showed strong cytoplasmic positivity for neurofilament protein (antibody RMDO-20 to NF-M) in a few of the tumor cells and in some of entrapped normal neurons. A reticulin stain showed minimal fibrosis. A CD31 immunostain highlighted the endothelial cells in tumor blood vessels, similar to the CD34, and also did not mark the tumor cells. Finally, a Nestin immunostain was strongly positive (Fig 11). What is the diagnosis?